In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in the surface of a substrate. There are known various techniques for forming such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper film is formed in the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as an oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer 1a in which semiconductor devices are formed, which is formed on a semiconductor base 1. Contact holes 3 and trenches 4 for interconnects are formed in the insulating film 2 by the lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the entire surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 1B, copper plating is performed onto the surface of the substrate W to fill the contact holes 3 and the trenches 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact holes 3 and the trenches 4 for interconnects and the surface of the insulating film 2 lie substantially on the same plane. An interconnection composed of the copper film 6 as shown in FIG. 1C is thus formed.
Components in various types of equipments have recently become finer and have required higher accuracy. As sub-micro manufacturing technology has commonly been used, the properties of materials are largely influenced by the processing method. Under these circumstances, in such a conventional machining method that a desired portion in a workpiece is physically destroyed and removed from the surface thereof by a tool, a large number of defects may be produced to deteriorate the properties of the workpiece. Therefore, it becomes important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of an altered layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
An electrolytic processing method that utilizes an ion exchanger has been developed. According to this method, an ion exchanger mounted on a processing electrode and an ion exchanger mounted on a feeding electrode are allowed to be close to or into contact with the surface of a workpiece. A voltage is applied from a power source to between the processing electrode and the feeding electrode while a liquid, such as ultrapure water, is supplied from a fluid supply section to between the processing and feeding electrodes and the workpiece, thereby carrying out removal processing of the surface layer of the workpiece.
FIG. 2 schematically shows a conventional electrolytic processing apparatus generally employed for such electrolytic processing. The electrolytic processing apparatus includes a processing electrode 52 and an ion exchanger 54 that is mounted on the processing electrode 52. Depending upon the material of a workpiece W, the processing electrode 52 is connected to the cathode or the anode of a power source 56, and the workpiece W is connected to the opposite pole, and the workpiece W is utilized as a feeding electrode. FIG. 2 shows the case where the processing electrode 52 is connected to the cathode of the power source 56 and the workpiece W is connected to the anode of the power source 56. The processing electrode 52 concentrates e.g. OH− ions, in an electrolytic solution capable of dissolving the atoms of the to-be-processed surface WA of the workpiece W, at the to-be-processed surface WA closed to the processing electrode 52 to cause a reaction between the atoms of the workpiece W and OH− ions, thereby processing the workpiece W. In the case of a semiconductor substrate W, a film of a conductive material formed in the substrate surface WA is removed by the processing electrode 52 in order to form semiconductor interconnects or contacts.
According to the conventional electrolytic processing apparatus, an ion exchanger for use in such electrolytic processing is tight on the exposed surface of a processing electrode or a feeding electrode, and is fixed on the electrode or at a peripheral portion of e.g. a support that supports the electrode, usually by screwing or using an adhesive tape or the like at a peripheral portion of the ion exchanger.
In recent years, as interconnects of the circuit to be formed in a semiconductor substrate has become finer with higher integrated density of the semiconductor device, it is desired to improve the flatness of the processed surface of the semiconductor substrate. Therefore, there is a demand for a technique that can improve the uniformity of the processing rate over the entire to-be processed surface.
Metals of the platinum group or their oxides have become candidates for an electrode material for use in forming a capacitor, which utilizes a high dielectric material, on a semiconductor substrate. Among them ruthenium, because of its good film-forming properties and good processibility for patterning, is being progressively studied as a feasible material.
A ruthenium film can be formed on a substrate generally by sputtering or CVD. In either method, deposition of the ruthenium film on the entire front surface of a substrate, including the peripheral region, is carried out. As a result, a ruthenium film is formed also in the peripheral region of the substrate and, in addition, the back surface of the substrate is unavoidably contaminated with ruthenium.
The ruthenium film formed on or adhering to the peripheral region or back surface of a substrate, i.e. the non-circuit region of the substrate, is not only unnecessary, but can also cause cross-contamination during later transfer, storage and various processing steps of the substrate whereby, for instance, the performance of a dielectric material can be lowered. Accordingly, during the process for forming a ruthenium film or after carrying out some treatments of the formed ruthenium film, it is necessary to completely remove the unnecessary ruthenium film. Further, in the case of using ruthenium as an electrode material for forming a capacitor, a step for removing part of a ruthenium film formed on the circuit region of a substrate is needed.
According to the conventional electrolytic processing apparatus as shown in FIG. 2, however, because of unevenness of the electric current value due to the shape of the processing electrode 52 or to the influence of the reaction products or gas bubbles generated during processing, the processing rate is likely to be uneven in the to-be-processed surface WA.
On the other hand, chemical mechanical polishing (CMP), for example, generally necessitates a complicated operation and control, and needs a considerably long processing time. In addition, a sufficient cleaning of a substrate must be conducted after the polishing treatment. This also imposes a considerable load on the slurry or cleaning liquid waste disposal. Accordingly, there is a strong demand for omitting CMP entirely or reducing a load upon CMP. Also in this connection, it is to be pointed out that though a low-k material, which has a low dielectric constant, is expected to be predominantly used in the future as a material for the insulating film of a semiconductor substrate, the low-k material has a low mechanical strength and therefore is hard to endure the stress applied during CMP processing. Thus, also from this standpoint, there is a demand for a technique that enables the flattening of a substrate without giving any stress thereto.
Further, a method has been reported which performs CMP processing simultaneously with plating, viz. chemical mechanical electrolytic polishing. According to this method, the mechanical processing is carried out to the growing surface of a plating film, causing the problem of denaturing of the resulting film.
Further, though it is desired that an ion exchange for use in electrolytic processing be tightly fixed on the exposed surface of a processing electrode or a feeding electrode, as described above, in order to ensure evenness of the processing accuracy, it has practically been difficult to keep the ion exchanger fixed tightly on the electrode.
Thus, when continuing electrolytic processing while an ion exchanger is fixed on an electrode by screwing or with an adhesive tape, the fixing of ion exchanger is likely to become incomplete, so that the ion exchanger can move easily, impairing evenness of the processing accuracy.